text: Spiciness, caused by the chemical Capsaicin, appears to be an adaptation of plants to prevent mammals from eating their fruit/peppers. Birds, which are immune to Capsaicin, appear to be the intended consumer of these fruits, which is understandible considering they fly and can, thusly, disperse the seeds over a wider area. As to why some humans have developed a fondness for Capsaicin, there's some evidence that Capsaicin inhibits bacterial growth and, in this way, helps preserve freshness in food. Spicy food is less likely to make you sick than non-spicy food due to this quality.